Australia's New South Wales (NSW) has reverted to making the wearing of masks mandatory on public transport in Sydney, as a cluster of the highly infectious coronavirus Delta variant expanded to a fourth person, Reuters reports.

Authorities said all planned outdoor events with good Covid-19 safety plans can proceed in the country's largest city.

The latest cluster, the first in the state in more than a month, was traced back to a driver who occasionally used to transport overseas airline crew.

Health officials said the latest case likely picked up the virus through minimal contact with an infected person in a Sydney shopping centre. “We don't want people panicking but at the same time we want everyone on high alert,” NSW state Premier Gladys Berejiklian told reporters in Sydney.